Regional and local drivers of marine invasions: identifying key abiotic and biotic factors in the Gulf of Maine

Background/Question/Methods

Invasive species are changing the structure and functioning of marine ecosystems. Marine invasions require inoculation (human vector) into adequate abiotic (oceanographic) and biotic (predatory) habitats.  Recently non-native encrusting tunicates, such as Didemnum sp, Botrylloides violaceus, Botryllus schlosseri, Ciona intestinalis and others, have increased in abundance along the southern coast of Maine. We used seven estuaries and bays with different thermal regimes and freshwater discharges (salinities), and high to low ship traffic and low to high aquaculture activities with the objective of testing theories about what drives the inoculation, establishment and success of invasive tunicates at local and regional scales. For this we deployed hundreds of 100cm² settlement plates at 1m depth at 24 sites along each estuary and bay, from Piscataqua to Cobscook Bay (approx. 400 km) that were photographed every 2-3 weeks. Abundance in percent cover and diversity were determined for each station. Temperature, salinity and depth were determined by deploying temperature loggers, salinity sondes and nautical charts, respectively. In the Damariscotta estuary, caging and laboratory experiments were designed to test the role of local predator over the abundance of common invasive ascidians at local scale

Results/Conclusions

We found diversity and abundance of non-native ascidians greatest in the Damariscotta and Piscataqua estuaries, where different tunicate species dominate different sites. Both areas have the warmest summer temperatures which may promote survival, colonization and growth. In contrast, estuaries with higher freshwater discharges and colder summer temperatures have significant lower abundances and diversity of fouling invasive species. We suggest that areas with colder temperatures and had pulses of low salinity water that may restrict the establishment and growth of the non-native tunicates despite potential inoculations from heavy ship traffic. Aquaculture might have promoted the introduction of one species and but might not be related to a regional pattern. At the regional scale, eastern sites in the Gulf of Maine, ruled by the cold-water, Eastern Maine Coastal Current, have significant lower abundances and diversity of invasive species. Benthic caging studies and laboratory observations in Damariscotta reveal that only solitary tunicates are susceptible to local predators.